LEXUS SC400 1991 Service Repair Manual

WIRING DIAGRAM
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±133
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NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals #20 ~ 40 of engine & ECT ECU and body ground.\
C
OK
P(2) Connect the Check Harness A.(See page TR±30)
(2) Turn ignition switch on.
Measure voltage between terminals #10 ~ 40 of engine
& ECT ECU and body ground.
Voltage: 10 ± 14 V
Go to step 3.
2Check ignition relay.
C
OK
Hint
PRemove ignition relay from R/B No.2.
Check continuity between terminals of ignition relay
shown below.
(2) Apply battery voltage between terminals of ignition relay shown below.
(2) Check continuity between terminals 1 and 2.
Replace ignition relay.
TR±134
±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

OKNG
OKNG
OKNG
3Check for open in harness and connector between terminal E01, E02 of eng\
ine & ECT ECU con-
nector and body ground (See page IN±27).
Repair or replace harness or connector.
4Check for open in harness and connector between ignition relay and injector, injector and en-
gine & ECT ECU (See page IN±27).
Repair or replace harness or connector.
5Check injectors.
C
OK
P
OK
C
Disconnect injector connector (See page FI±50).
Measure resistance of injector.
Resistance: 13.4 ± 14.2
at 20 C (68 F)
Check injection volume of injector (See page FI±52).

Injection volume
55±70 cc/15 sec. (3.4±4.3 cu in.)
Difference between each injector:
Less than 10 cc (0.6 cu in.)

Leakage
Fuel drop: Once drop or less per minute.
Replace injector.
Proceed to next circuit inspection shown on ma-
trix chart (See page TR±35).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±135
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Cold Start Injector Circuit
CIRCUIT DESCRIPTION
The cold start injector is used to maintain the engine startability when it\
is cold. The injection volume, i,e, the
length of time the injector is energized, is controlled by the ECU and t\
he cold start injector time switch.
During a cold start, when the starter turns the contacts in the cold start inj\
ector time switch close. Thus current
flows to the cold start injector coil, injecting fuel. At the same time, a bi\
metal in the heat coil is energized and
heats up. This soon causes the contacts to open, cutting off the current flow to the injector coil and stopping
fuel injection.
The injection duration of the cold start injector is determined by the cool\
ant temperature and the length of time
current flows to the heat coil. When the engine is warm, the contacts are opened by the bimetal and the cold
start injector does not operate.
When the engine is hard to start and the starter is operated continuously, heat coil (2) heats up the bimetal keep-
ing the contacts open to prevent spark plugs from becoming fouled, which is\
caused by the cold start injector
operation when the open contacts close again.
When the engine is started at a coolant temperature of 225C (72 5F) or lower, the cold start injector operation
time is controlled by the cold start injector time switch.
When the coolant temperature is in the normal temperature range 22 5C (72 5F) or higher, the contacts of the cold
start injector time switch are open and the time switch is off, instead, the ECU controls the operating time of the
cold start injector.
In this way, the CO and HC levels can be reduced while the engine is being started \
and the engine startability
is maintained. Control by the ECU ends when the coolant temperature reac\
hes 60 5C (140 5F).
TR±136±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

DIAGNOSTIC CHART
DIAGNOSTIC
CHART
Check voltage of cold start injector power
source.
Check cold start injector.
Check start injector time switch.Replace start injector time switch.
Repair or replace harness or
connector.
Replace cold start injector.
Check for open and short in harness and
connector between cold start injector and
start injector time switch.
Check fuel injection of cold start injector.
Proceed to next circuit inspection shown on
matrix chart (See page TR±35).
Check for open in harness and connector
between battery and ECU.
Replace cold start injector.
WIRING DIAGRAM
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±137
WhereEverybodyKnowsYourName

NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminal STJ of engine & ECT ECU connector and body gro\
und.
C
OK
Hint
PConnect to the Check Harness A.
(See page TR±30)
Disconnect start injector time switch connector.
Disconnect engine & ECT ECU connector.
Measure voltage between terminal STJ of engine &
ECT ECU connector and body ground when ignition
switch is turned to STA.
10 ± 14 V
Go to step 3.
2Check resistance of cold start injector.
C
OK
Hint
PRemove cold start injector (See page FI±27).
Measure resistance between terminals of cold start in-
jector.
Resistance: 2 ± 4
at 20
C (68
F)
Replace cold start injector.
Check and repair harness and connector between
engine & ECT ECU and cold start injector, cold
start injector and battery.
TR±138±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

OKNG
OKNG
OKNG
3Check start injector time switch.
C
OK
PRemove start injector time switch (See page FI±88).
Measure resistance between terminals shown below.
Replace start injector time switch.
4Check harness and connector between cold start injector and start inject\
or time switch (See
page IN±27 ).
Repair or replace harness or connector.
5Check fuel injection of cold start injector.
C
OK
PRemove cold start injector (See page FI±27).
(2) Check fuel injection of cold start injector (See page FI±33).
(2) Check fuel leakage of cold start injector (See page FI±34).
(2) Fuel is injected normally.
(2) Fuel leakage is less than one drop per minute.
Notice:
Perform this check within the shortest possible
time.
Repair or replace cold start injector.
Proceed to next circuit inspection shown on ma-
trix chart (See page TR±35).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±139
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ISC Valve Circuit
CIRCUIT DESCRIPTION
The ISC valve is provided on the intake air chamber and in-
take air bypassing the throttle valve is directed to the ISC
valve through a hose.
A step motor is built into the ISC valve. It consists of four
coils, the magnetic rotor, valve shaft and valve. When cur-
rent flows to the coils due to signals from the ECU, the rotor
turns and moves the valve shaft forward or backward,
changing the clearance between the valve and the valve
seat.
In this way the intake air volume bypassing the throttle valve
is regulated, controlling the engine speed.
There are 125 possible positions to which the valve can be
opened.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
Check operation for ISC valve.Replace ISC valve.
Repair or replace harness or
connector.
Proceed to next circuit inspection shown on
matrix chart (See page TR±35).
Check harness and connector between EFI
main relay, ISC valve and ECU.
TR±140±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

OKNG
OKNG
INSPECTION PROCEDURE
1Check ISC value.
C
OK
P
P
OK
C
Disconnect ISC connector.
Measure resistance between terminals shown below.
Remove ISC Valve
(2) Connect the battery positive lead to terminals B1and B2, and the negative lead to terminals
S1±S2±S3±S4 in that order.
(2) Connect the battery positive lead to terminals B1 and B2, and the negative lead to terminals
S4±S3±S2±S1 in that order.
(2) The valve moves in the closing direction.
(2) The value moves in the opening direction.
Replace ISC valve.
2Check for open and short in harness and connector between EFI main relay and \
ISC valve, ISC
valve and engine & ECT ECU (See page IN±27).
Repair or replace harness or connector.
Proceed to next circuit inspection shown on ma-
trix chart (See page TR±35).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±141
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Fuel Pressure Control VSV Circuit
CIRCUIT DESCRIPTION
The ECU turns on a VSV (Vacuum Switching Valve)
to draw the air into the diaphragm chamber of the
pressure regulator if it detects that the temperature of
the coolant is too high during engine starting.
The air drawn into the chamber increases the fuel
pressure to prevent fuel vapor lock at high engine
temperature in order to help the engine start when it
is warm.
Fuel pressure control ends approx. 100 secs. after
the engine is started.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
Check operation for fuel pressure control
VSV.Replace fuel pressure control VSV.
Repair or replace harness or
connector.
Proceed to next circuit inspection
shown on matrix chart (See page
TR±35).
Check for open and short in harness and
connector between main relay and ECU.
Check voltage of VSV power source.
Check and replace ECU.
WIRING DIAGRAM
TR±142±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName